Metal forming or stamping presses and systems for monitoring the loads on such presses are well known in the art. Looking first at FIG. 1, press load monitoring systems typically comprise one or more strain sensors 5 mounted to the frame 10 of a press 15 for measuring the strain on the frame of the press, and electronic means 20 for (a) calculating the load on the frame of the press from the output of strain sensors 5, (b) comparing the calculated load on the frame of the press to predetermined set points, and (c) automatically shutting down the press if the calculated load on the frame of the press is outside the limits of those predetermined set points. See, for example, U.S. Pat. Nos. 4,048,848, 4,062,055, 4,171,646, and the references cited therein for typical press monitoring systems.
Due to variations in manufacture and operating conditions, both the presses and the press load monitoring systems must generally be initially calibrated before use; the presses must be calibrated to ensure that they strike the workpiece with the desired force, and the press load monitoring systems must be calibrated to ensure that they properly calculate the actual load on the frame of the press from the output of strain sensors 5.
In a constant speed press, the press and the system for monitoring the load on the press are typically initially calibrated as follows. First, a load cell is positioned in the press in place of the normal tooling. This load cell is adapted to register and display the actual impact force generated by the press on the load cell, e.g. 97 tons. The press is started up, the output of the load cell is monitored, and the press is adjusted as necessary until the press is in fact striking the load cell with the desired force, e.g. 100 tons.
After the press has been so calibrated, the press load monitoring system must then be calibrated. Such calibration essentially consists of determining a correct calibration factor (measured in tons/volt) by which the output of strain sensors 5 (measured in volts) must be multiplied in order to correctly yield the actual load being exerted on the frame of the press. More specifically, the calibration factor for the press is computed by utilizing the following equation: ##EQU1## By way of example, suppose press 5 has just been calibrated so that it is known to be operating at its rated capacity of 100 tons. Suppose also that while the press was striking the load cell with an impact force of 100 tons, strain sensors 5 were showing an output of 5 volts. Then, by applying Equation (1) above and solving for the calibration factor, the appropriate calibration factor for the press is computed, i.e., ##EQU2## EQU 100 tons=calibration factor.times.5 volts (1b) EQU calibration factor=100 tons/5 volts (1c) EQU calibration factor=20 tons/volt (1d)
This calibration factor is then stored by the press load monitoring system to be thereafter used whenever the actual load on the frame of the press needs to be determined from the output of strain sensors 5. More specifically, once the press and the press load monitoring system have been calibrated in the foregoing manner, and the press load monitoring system has had its predetermined set points set, e.g. it might be given a "high" set point of 120 tons and a "low" set point of 80 tons, the load cell is replaced by normal tooling and operation of the press is commenced. Thereafter, electronic means 20 monitor the output of strain sensors 5, continually calculating the load on the frame of the press using Equation (1) discussed above, the calibration factor previously determined and the output of strain sensors 5, e.g. if strain sensors 5 were reporting an output of 4.5 volts and the press had a calibration factor of 20 tons/volt, electronic means 20 would calculate a load level of 90 tons on the frame of the press, or if strain sensors 5 were reporting an output of 5.5 volts and the press had a calibration factor of 20 tons/volt, electronic means 20 would calculate a load level of 110 tons on the frame of the press. So long as the load level computed by electronic means 20 (utilizing Equation (1) discussed above, the calibration factor previously determined and the output of strain sensors 5) remains between the predetermined "high" and "low" set points, electronic means 20 allow the press to continue operation uninterrupted; however, as soon as the load level computed by electronic means 20 falls outside the "high" and "low" set points, e.g. above 120 tons or below 80 tons, electronic means 20 will automatically shut down the press.
In a variable speed press, the situation is significantly more complex. This is because during the operation of the press, the frame of the press typically has mechanical resonances in it from the action of the successively opening and closing press members. These mechanical resonances create a periodic "ringing" in the frame of the press that is picked up by the aforementioned strain sensors during operation of the press load monitoring system and is included in the total strain levels reported by the strain sensors to the electronic means 20. It has been found that, for a given press, the strain component attributable to "ringing" in the press frame tends to vary with the speed of the press; at slow press speeds, e.g. 200 strokes per minute (200 "spm"), the strain component from "ringing" tends to be relatively small relative to the total strain levels reported by the strain sensors, whereas at high press speeds, e.g. 1200 spm, the strain component from "ringing" tends to be fairly large relative to the total strain levels reported by the strain sensors. It has also been found that, for a given press, the strain component (as a percentage of load) attributable to "ringing" tends to be fairly constant for a given press speed. See FIGS. 2A and 2B, which show representative strain sensor outputs for a typical variable speed press operating at 200 spm and 1200 spm, respectively.
On account of the foregoing, it is significantly more difficult to properly utilize a press load monitoring system with a variable speed press. More specifically, suppose both the variable speed press and the press load monitoring system are calibrated in the foregoing manner at a relatively low press speed, e.g. 200 spm. The load cell is placed on the press, the press is turned on, and the output of the load cell is monitored and the press is adjusted as necessary until the press is impacting the load cell with the desired force, e.g. 100 tons. With the press operating at its known rating of 100 tons, the output of strain sensors 5 is read and, through the use of Equation (1) above, the appropriate calibration factor for the press is computed, e.g. if strain sensors 5 were reporting an output of 5 volts while the press was generating 100 tons, the calibration factor for the press would once again be 20 tons/volt. Now if the speed of the press should be increased to a relatively high speed, e.g. 1200 spm, while the load cell is left in the press, an interesting phenomena will be observed: the load cell's output might change slightly, e.g. it might drop from 100 tons to 99.5 tons, to reflect certain speed-related changes occurring in the operation of the press such as a tightening of bearings, etc., while at the same time the output of strain sensors 5 will likely change fairly dramatically, e.g. it might increase from 5 volts to 6.5 volts. Electronic means 20 would read this 6.5 volt output, apply the previously determined calibration factor of 20 tons/volt as per Equation (1) above, and conclude that the load on the frame of the press had suddenly increased from 100 tons to 130 tons--which is, of course, in direct contradiction to the current output of the load cell, which is still showing a load of approximately 100 tons. Clearly the increase in the output of strain sensors 5 does not reflect an actual increase in the force generated by the press during its power stroke; in fact, the increase in the output of strain sensors 5 is generally directly traceable to a corresponding increase in the strain component relating to "ringing".
The fact that the strain component attributable to "ringing" varies with the speed of the press presents something of a problem for the press load monitoring system: if the calibration factor for the press is initially calculated at a relatively low rate of speed, e.g. 200 spm, and the "high" and "low" set points appropriately set, e.g. at 120 tons and 80 tons, respectively, and the press is then run at its high rate of speed, e.g. 1200 spm, "ringing" in the press frame will cause electronic means 20 to calculate an erroneously high load level, e.g. 200 tons instead of 100 tons, whereupon the press load monitoring system will then generally erroneously shut down the system. On the other hand, it will also be appreciated that if the calibration factor for the press should be initially calculated at a relatively high rate of speed, e.g. 1200 spm, (i.e., so that calibration is done while the strain sensors are reading a sizeable additional strain component attributable solely to "ringing"), and the "high" and "low" set points set, e.g. at 120 tons and 80 tons, respectively, and the press is then run at its low rate of speed, e.g. 200 spm, the absence of sizeable strain components attributable to "ringing" will then cause electronic means 20 to calculate an erroneously low load level, e.g. 50 tons instead of 100 tons, whereupon the press load monitoring system will then generally also incorrectly shut down the system.
Clearly, speed-related variations in the strain component attributable to "ringing" present serious obstacles to the use of conventional press load monitoring systems with variable speed presses.
The foregoing problem of using conventional press load monitoring systems with variable speed presses is rendered particularly difficult since in practice press operators tend to start a variable speed press at a relatively slow speed, e.g. 200 spm, run it at that slow speed for a brief period to be sure that everything is in order, and then rapidly increase the speed of the press until it reaches a high standard operating speed, e.g. 1200 spm. The problem of properly compensating for speed-related variations in the strain component attributable to "ringing" is aggravated where the speed of the press is frequently and rapidly changed.
In practice, press operators tend to either calibrate the press load monitoring system for the high standard operating speed of the press which is to be ultimately achieved, e.g. they calibrate the press load monitoring system at 1200 spm, or they calibrate the press load monitoring system at the low rate of speed, e.g. 200 spm, and then try to continually adjust the controls of the press load monitoring system manually as the speed of the press changes in an effort to properly compensate for speed-related variations in the strain component relating to "ringing". The former technique guarantees that the press load monitoring system will properly compensate for "ringing" at the high standard operating speed of the press, but such proper compensation at the high standard operating speed is generally at the expense of incorrectly compensating for "ringing" at lower press speeds. As a result, the former technique can totally negate the advantage of a press load monitoring system at these lower press speeds. In addition, the former technique generally requires that an extremely low--or even nonexistent--"low" set point be set to avoid having the press load monitoring systems erroneously shut down the press at low rates of speed. The latter technique, on the other hand, is a nuisance which requires constant operator attention and which can lead to serious problems if the press load monitoring system is not properly readjusted every time the speed of the press is changed.